The present invention relates to a method and apparatus for the froth flotation separation of minerals and other particulate matter and more particularly relates to a method and apparatus for the concentration and beneficiation of carbonaceous matter, especially coal, by column flotation.
Valuable minerals are commonly found in nature mixed with relatively large quantities of unwanted gangue materials, and as a consequence it is usually necessary to beneficiate the ores in order to concentrate the mineral content thereof. Mixtures of finely divided mineral particles and finely divided gangue particles can be separated and a mineral concentrate obtained therefrom by well known froth flotation techniques. Broadly speaking, froth flotation involves conditioning an aqueous slurry or pulp of the mixture of mineral and gangue particles with one or more flotation reagents which will promote flotation of either the mineral or the gangue constituents of the pulp when the pulp is aerated. The conditioned pulp is aerated by introducing into the pulp a plurality of minute air bubbles which tend to become attached either to the mineral particles or the gangue particles of the pulp, thereby causing these particles to rise to the surface of the body of pulp and form a float fraction which overflows or is withdrawn from the flotation apparatus.
Coal is an extremely valuable natural resource in the United States because of its relatively abundant supplies. It has been estimated that the United States has more energy available in the form of coal than in the combined natural resources of petroleum, natural gas, oil shale, and tar sands. Recent energy shortages, together with the availability of abundant coal reserves and the continuing uncertainties regarding the availability of crude oil, has made it imperative that improved methods be developed for converting coal into a more useful energy source.
Regardless of the form in which the coal is ultimately employed, the coal or coal combustion products must be cleaned because they contain substantial amounts of sulfur, nitrogen compounds and mineral matter, including significant quantities of metal impurities. During combustion these materials enter the environment as sulfur dioxides, nitrogen oxides and compounds of metal impurities. If coal is to be accepted as a primary energy source, it must be cleaned to prevent pollution of the environment either by cleaning the combustion products of the coal or by cleaning the coal itself prior to burning.
Accordingly, physical as well as chemical coal cleaning (beneficiation) processes have been explored. In general, physical coal cleaning processes involve pulverizing the coal to release the impurities, wherein the fineness of the coal generally governs the degree to which the impurities are released. However, because the costs of preparing the coal rise exponentially with the amount of fines to be treated, there is an economic optimum in size reduction. Moreover, grinding coal even to extremely fine sizes may not be effective in removing all the impurities. Based on the physical properties that effect the separation of the coal from the impurities, physical coal cleaning methods are generally divided into four categories: gravity, flotation, magnetic and electrical methods. In contrast to physical coal cleaning, chemical coal cleaning techniques are in a very early stage of development. Known chemical coal cleaning techniques include, for example, oxidative desulfurization of coal (sulfur is converted to a water-soluble form by air oxidation), ferric salt leaching (oxidation of pyritic sulfur with ferric sulfate), and hydrogen peroxide-sulfuric acid leaching.
Particularly desirable coal beneficiation processes are disclosed in relatively recently issued U.S. Pat. Nos. 4,412,843, 4,347,126, 4,347,127 and 4,514,291.
U.S. Pat. No. 4,412,843 discloses a froth flotation process whereby coal particles are made highly hydrophobic and oleophilic by utilizing surface treating chemicals. U.S. Pat. Nos. 4,347,126 and 4,347,127 disclose improved coal beneficiation processes, which may be used in conjunction with the process disclosed in U.S. Pat. No. 4,412,843 and which advantageously utilize spray nozzles in the flotation process. In turn, U.S. Pat. No. 4,514,291 discloses a coal beneficiation process which utilizes a spiral type spray nozzle to provide further advantages over those provided by the previously disclosed processes.
Various types of flotation systems are available including conventional flotation which relies on a draining froth bed to separate the mineral-laden bubbles from the slurry and column flotation which operates as a counter-current system with the bubbles rising through a downward stream of wash-water.
Literature references comparing the various aspects of conventional flotation with column flotation are available. Some of these include Mathieu, "Comparison Of Flotation Columns With Conventional Flotation For Concentration Of a Molybdenum Ore," Extractive Metallurgy, pp. 1-5, (1972), Sastry et al., "Theoretical Analysis Of A Countercurrent Flotation Column." Transactions SME/AIME, Vol. 247, No. 1, pp, 46-52 (March, 1970) and Dell, "Column Flotation Of Coal--The Way To Easier Filtration" (1976). Patents which disclose various column flotation methods and apparatus include U.S. Pat. Nos. 4,436,617, 3,371,779, 3,339,730, 3,298,519, 2,897,144, 2,047,989, 1,367,332, 1,314,316, 1,223,033, German Pat. No. 213,141 and Swedish Pat. No. 121,991.
While it is obvious from the foregoing that enormous efforts have been made to beneficiate ores, particularly coal, further work and improvements are still necessary and desirable particularly before coal and other solid carbonaceous fuel sources will be accepted on a wide scale as primary sources of energy.